Dynamics of Magnetic Structures during a Magnetospheric Substorm

The Earth's magnetosphere and the ambient interplanetary environment can create favorable conditions for the nonlinear process of energy release in the form of changes in the topology of the magnetic field and current systems, particle acceleration, wave generation, and sharp gradients in the parameters inherent to a substorm phenomenon. Initially, early studies substantiated the role of variations in the solar wind parameters as a key factor responsible for the onset of a magnetospheric substorm; however, this factor was later shown not to be decisive. Over several decades, continuously improving methods for designing measurement tools and analyzing data helped to identify the processes that accompany the substorm phenomenon and describe them both qualitatively and quantitatively. However, there is no consensus in understanding the scenario of substorm development stepwise. The purpose of the research is to determine the propagation and orientation features of transients (fronts) in the current sheet of the Earth's magnetotail during a substorm. To do this, the magnetic field measurements obtained by the four spacecraft of the Cluster II mission for July 20, 2013, are analyzed. During this event, spacecraft were located on the night side of the Earth's magnetosphere and recorded changes in the geomagnetic field during the magnetospheric substorm. We used the single-spacecraft method for finding the minimum variance of the magnetic field and multispacecraft timing analysis involving cross-correlation of time series. The first method allows finding the direction of the normal to the structure under study; the second method makes it possible to find the direction and absolute value of its propagation velocity. The results of the study show that, with the development of substorms, the fronts that move towards the Earth exhibit a decrease in the propagation velocity and a significant degree of curvature. The first effect (a decrease in the propagation velocity of the fronts) indicates a decrease in the energy reserve of the current sheet for the generation of such transients, and the second effect (a significant degree of curvature) indicates the azimuthal localization of the front.